So what are we talking about? Smart wall systems could comprise technologies ranging from infrared and laser-enhanced cameras, drones, sensors and radar to artificial intelligence. There's also imaging technology that scans vehicles for drugs as they pass through official border crossings.

But these approaches could too easily violate our privacy, security and civil liberties, according to tech companies and the American Civil Liberties Union. The ACLU described security technologies as "ineffective" and warned that "programs intended for border security purposes have had a tendency to expand toward the rest of the country."

Here are the most likely technologies that could be used on the border in place of, or in addition to, a wall.

Sensors

Sensor technology is now small enough to embed in a car, allowing it to power self-driving test vehicles.

Advocates say today's sensors could be used on the border.

One company, Quanergy, says one of its products relies on lidar, which uses pulsed light to measure distances and generate detailed images, to scan for movements. Quanergy says its products can tell the difference between a person and an animal.

Given that the border stretches 1,950 miles, the project might require more than 15,000 sensors. At an estimated $250 per sensor, the price tag could amount to $3.7 million, or less than 1 percent of Trump's border wall ask.

Then there's Anduril, a 2-year-old startup that's building sensor-packed towers filled with radar, laser-enhanced cameras and communications antennas. Together, the sensors, cameras and antennas can detect objects from two miles away and identify the difference between people and animals, the company says.

Anduril is headed by Palmer Luckey, the 26-year-old co-founder of Oculus VR . Now in his second act, he's hoping to "rebuild the bridge between the tech and defense communities."

His technology, called Lattice, is already being tested by US Customs and Border Protection (CBP). In a 10-week span, it helped agents catch 55 unauthorized border crossers and seize 982 pounds of marijuana, according to a 2018 Wired profile of the company.

Anduril's and Quanergy's technologies could potentially augment an existing program created by Elbit Systems, an Israeli company that's reportedly built 43 towers in Arizona carrying radar as well as daylight and infrared cameras that can capture images from 7.5 miles away, according to a report by Bloomberg. The contract, awarded in 2014 for $145 million, has been called a key piece of technology by CBP.

Drones

Drones can use combinations of cameras, lasers and heat sensors. They've also been tested in war zones by the US military and by firefighters .

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The US Customs and Border Protection agency uses drones like this Predator for surveillance flights near the Mexican border.

"The biggest difficulty with drones is positioning them to surveil the border," Cato's report said, noting that issues such as bad weather, crashes and operation costs have plagued the program.

Though CBP spent $255 million buying and operating drones between 2013 and 2016, apprehensions have risen by less than 1 percent, Cato estimates, at a cost of $32,000 per capture. Captures not involving drones cost an average of $9,000, according to Cato.

CBP didn't respond to a request for comment.

Artificial intelligence

The tech industry's buzziest buzzword is already being used to help identify people in your photographs, recommend new beauty products and weed out bad behavior on the internet. And there are companies who say AI can help secure the border as well.

Cogniac offers camera software that identifies people and objects. The company is backed by Google parent Alphabet and says its technology can work in real time with security cameras, cameras mounted on drones or even smartphones. 

So where is facial recognition in all this, you might ask?

That technology has two potential issues. First, it would be mostly practical in places like border crossings, where lots of people stream by cameras. Second, such systems aren't foolproof, and they carry a very real risk of abuse.

"When combined with ubiquitous cameras and massive computing power and storage in the cloud, a government could use facial recognition technology to enable continuous surveillance of specific individuals," Microsoft President Brad Smith wrote in an official Microsoft blog last year. "We must ensure that the year 2024 doesn't look like a page from the novel '1984.'"

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